“It is often said that we never get a second chance to make a first impression. This holds true in our writing. We may be witty, charming, and absolutely delightful company, but our readers don’t know that. They judge us by what we share with them—our written words. Have they been professionally polished to perfection?” --Linda Lane in What Makes a Book Marketable? #2.
As an editor, I’ve rejected stories because they contained too many grammatical errors or unresolved plot issues. The sloppy writing and poor editing turned me off to what might have been an interesting story. For me, poor grammar is a sign of an amateur writer. As for aspects of plotting, Chekov once said, “If a gun is loaded in Act One, it must be fired by Act Three.” An author can’t simply introduce Aunt Tilly at the beginning of chapter two and then ignore her for the rest of the scene. There must be a reason for her to in the book. If there isn’t, leave her out.
Even well-known writers fall into the trap of poor editing. I remember reading a book in a highly-regarded series of detective novels where the main character parks her car outside the murder’s home and leaves the keys in the ignition in case the guy comes home and she needs to get away fast. A few pages later, she’s trapped in the cellar and uses her car keys to loosen the hasps on the locked door leading to the backyard. Oops. I guess the author forgot to mention the MC carries two sets of keys. I haven’t read any more of the novels in the series.
In another novel -- again, part of a series -- the antagonist shoots a revolver just to experience what happens when the gun goes off. Later in the book, he points the gun at someone and hesitates as he wonders to himself what will happen when he pulls the trigger. Huh? Was it his twin who shot the gun earlier?
Careful editing is a must. Writers shouldn’t rely simply on their own editing skills -- or worse, relatives or close friends. They’re too close to their stories to find some mistakes. A scene that makes sense to the author may be a mystery to the reader because the author forgets to share something he knows that is pertinent with the reader. The same goes for grammatical errors. Often, the writer is simply too close to the prose to recognize these mistakes.
Here's another example of the importance of writing groups. I belong to three and find each one helpful in polishing my stories.